produced by the Polarizing Microscope. 163 



a plate of glass and cover them with a thin film of castor-oil, 

 which of course cannot be made parallel. By viewing them 

 in the polarizing apparatus, and making the field of view alter- 

 nately dark and luminous, we shall see the superior distinctness 

 of vision which is obtained by the use of polarized light- In 

 getting rid of the second plate of glass we get rid of its im- 

 perfections, both of surface and of substance, and we get rid 

 also of the spherical aberration which it introduces, and for 

 the removal of which we are obliged, when using high powers, 

 to make an adjustment in the object-glass. 



The preceding observations are of course applicable only 

 to those microscopic objects which depolarize light ; but there 

 is scarcely an animal or a vegetable fibre which does not pos- 

 sess this property. The minutest hair of the smallest animal 

 which I have been able to procure depolarizes light; and if a 

 case should occur where the depolarizing structure exists, and 

 could be rendered visible by doubling the thickness of the 

 fibre, we might obtain this effect by making the polarized light 

 pass twice through the fibre by reflexion, and thus exhibit 

 itself luminously on a dark ground. 



When microscopic objects have no action upon polarized 

 light, we can only remove the indistinctness produced from dif- 

 fraction by such an illumination as will cause the light to di- 

 verge from the object as if it were self-luminous. The method 

 of doing this was first described in the Edinburgh Journal of 

 Science, and afterwards more fully in my Treatise on the Mi- 

 croscope; but as this method has been confounded with Dr. 

 Wollaston's* method of illuminating microscopic objects by 

 a celebrated optician, M. Chevalier of Paris, I shall embrace 

 the present opportunity of showing that the two methods have 

 no resemblance whatever. 



In proposing a method of illuminating microscopic objects, 

 Dr. Wollaston's object was to get rid of the superfluous light 

 which gave indistinctness to microscopic vision, and not to 

 remove the evils arising from diffraction. He never even 

 mentions diffraction, or any other cause but ihai oi supojluons 

 light, as the origin of imperfect vision, arising from the usual 

 modes of illumination. He was, indeed, not aware that the 

 diffraction oi ihe light used for illuminating objects was the 

 evil to be corrected ; and he has accordingly neither corrected 

 it by his method, nor attempted to correct it. "In the illu- 

 mination of microscopic objects," he says, " whatever light is 

 collected and brow^ht to the eije beyond that which is fully com- 

 manded by the object-glasses, tends rather to impede than assist 

 distinct vision. My endeavour has been, to collect as much 

 • Philosophical Tiansactions, 1829, p. 9. 

 M2 



